This invention relates to a transmission-controlling system of a vehicle having both an automatic transmission (hereinafter abbreviated as "AT") and an anti-lock brake system (hereinafter abbreviated as "ABS").
In AT vehicles, divergence of a throttle valve of an engine and vehicle speed are detected, a proper speed gear is determined by using both data signals with reference to a predetermined change-speed graph, and the transmission is controlled to shift to the determined speed gear.
FIG. 2 shows an example of a change-speed graph for normal operation, in which vehicle speed is plotted as abscissa and divergence of throttle valve as ordinate. Solid line I shows a 3.fwdarw.4 shift-up curve (a curve showing the time of shifting up the gear from 3rd speed gear to 4th speed gear). When a vehicle droven with a 3rd speed gear acquires an increased vehicle speed in a manner of crossing the shift-up curve I as shown by the arrow A in FIG. 2, the transmission is controlled to select the 4th speed gear.
Dotted line II shows a 4.fwdarw.3 shift-down curve (a curve showing the time of shifting down the gear from 4th speed gear to 3rd speed gear). When a vehicle droven with a 4th speed gear acquires a decreased vehicle speed in a manner of crossing the shift-down curve II as shown by the arrow B in FIG. 2, the transmission is controlled to select the 3rd speed gear.
Although FIG. 2 shows only two curves relating to 3rd and 4th speeds, there exist curves relating to 1st speed, 2nd speed, etc.
On the other hand, AT vehicles for business use suffer more abrasion of a brake lining than AT passenger vehicles because they are droven in a heavily loaded state with putting on brake many times. Therefore, in order to reduce the abrasion, a so-called "automatic engine brake control" has been considered which functions to automatically acquire engine brake when application of brake is necessary. To be specific, the automatic engine brake control is to control so that, when a throttle valve position of a vehicle droven with a 4th speed gear at a speed within a predetermined range is restored to the idle position, the speed gear is automatically shifted down to the 3rd speed gear.
FIG. 3 shows an example of a change-speed graph for acquiring automatic engine brake. Symbols used in FIG. 3 correspond to those used in FIG. 2. As can be immediately seen by comparing with FIG. 2, both solid line I and dotted line II in FIG. 3 make a turn to the right at a certain small throttle valve divergence in a direction in which vehicle speed increases and, after advancing a certain distance in a direction almost parallel to the abscissa, they again make a turn in a downward direction almost parallel to the ordinate. Change-speed graphs of such patterns can attain the automatic engine brake control.
For example, when a throttle valve of a vehicle droven at a speed within the range of the change-speed curve II being almost parallel to the abscissa is restored to the idle position (at which the divergence of the throttle valve is zero on the ordinate), the throttle valve divergence crosses the change-speed curve II from above (4th speed side) to below (3rd speed side) as shown by the arrow C, thus the transmission being controlled to shift down to the 3rd speed gear to acquire engine brake. In the normal operation shown by FIG. 2, the divergence change as shown by the arrow C would not cross the change-speed curve II, and hence no shifting down to the 3rd speed gear takes place. Accordingly, a required brake must be obtained by pedaling a foot brake, leading to a rapid abrasion of a brake lining.
On the other hand, there is known ABS as a system for not causing slip of wheels upon putting on brake. This is a system which detects the rotating speed of wheels and, when wheels are nearly locked, automatically reduces the braking power to thereby prevent wheels from being locked. This ABS is also often loaded on AT vehicles.
In the conventional transmission-controlling system for acquiring automatic engine brake, this system functions independently from ABS. Therefore, there have been a problem that ABS function might be damaged and a problem that a serious shock is produced upon shifting gear.
When a driver removes his foot from the accelerator pedal during driving a vehicle at a 4th speed, the throttle valve is restored to the idle position, and the transmission-controlling system for acquiring engine brake functions to shift down the gear to a 3rd speed gear. Thus, each wheel receives a braking power due to engine brake. However, if this happens during the period wherein ABS is in operation (to reduce the braking power on each wheel), ABS function might be damaged.
In addition, when automatic engine brake is acquired to shift down the gear to the 3rd speed gear upon ABS being in operation, a serious shock is produced upon shifting of gear if the rotation number on the engine side is greatly different from that on the transmission side. If rotating wheels are in contact with a split-surfaced road (for example, a road separated into a frozen surface-having portion and a non-frozen surface-having portion), vehicle stability in the running direction is damaged by the shock to possibly cause a traffic accident.
The subject of this invention is to solve the abovedescribed problems.